Disk Fragmentation

What's the best way to find out if disk fragmentation on Windows 2000 Server is affecting SQL Server performance?

If disk fragmentation is shown to be a cause of performance problems, what are the recommendations for a disk fragmentation strategy? eg. use the win 2000 built in disk defrag utility or buy a 3rd party product like DiskKeeper? How much of an overhead is a product like DiskKeeper that defrags in the background?

Clivewould running a trace after defragmentation, and running it again when the disk gets fragmented aid in this? you could use the built in defragger, but i would run it when the server is at it's quietest.